1. Field of the Invention
The present invention relates to an anaesthetic delivery system and in particular to a system of the type wherein gaseous anaesthetic expired by a patient is re-used by introducing it into gas intended to be inspired by the patient.
2. Description of the Prior Art
A patient anaesthetic delivery system is described in U.S. Pat. No. 5,471,979 wherein fresh respiration gas passes through an adsorption filter to take up adsorbed anaesthetic, which was collected from gas expired by the patient that also passed through the filter. In this manner expensive anaesthetic, which would otherwise be lost, can be reused. This system has a common inspiration/expiration gas pathway for the delivery and the discharge of anaesthetic and respiration gases to and from the patient and the adsorption filter is disposed in this common pathway, for the adsorption and desorption of anaesthetic gas. A second pathway, which bypasses the adsorption filter, is also provided for the delivery of anaesthetic and/or respiration gases to the patient without passing through the adsorption filter. Practically, the efficiency (i.e. the percentage of expired anaesthetic retained by the filter) of the adsorption filter will be required to be fixed at less than 100% (typically 80%). Some anaesthetic within the expired gas will then always pass through the filter to be lost from the respiration system. This is necessary in order to make the system more responsive to user required changes in the concentration of anaesthetic to be delivered to the patient.